For years, Col. Gen. Mikhail Mizintsev was an obscure figure in Russia's sprawling military leadership. But over the past week, he's become well-known under a startling moniker: "The butcher of Mariupol."

Ukrainian officials and activists have accused Mizintsev of orchestrating a devastating siege of the southern Ukrainian port city that, according to its mayor, has killed thousands of civilians and leveled residential buildings.

"This is Mikhail Mizintsev. He is leading the siege of Mariupol. . . . He has huge experience of destroying cities in Syria," Oleksandra Matviichuk, head of Ukraine's Center for Civil Liberties, tweeted last week over a photo of the 59-year-old general, a man with close-cropped gray hair and pale blue eyes.

Mizintsev, who heads Russia's National Defense Control Center, was placed under sanctions Thursday by the British government, which said he was "linked to the planning and execution of the bombardment of Mariupol, among other key Russian military operations against Ukraine."

The Russian military officer had been dubbed "the butcher of Mariupol," Britain's Foreign Office said in a statement.

But despite his newfound notoriety, some Russia-watchers were surprised to see Mizintsev singled out. Previously, he did not have much of a reputation despite decades of service, experts said.

"I honestly don't understand all this - I don't think he has any operational command responsibility here, and he's not got any particular reputation as martinet or thug that I know of," said Mark Galeotti, an honorary professor at University College London who studies the Russia's armed forces.

Jeffrey Edmonds, former director for Russia on the National Security Council and a senior analyst at the CNA think tank in Washington, said he knew little about Mizintsev and had never interacted with him.

Keir Giles, a Russia expert at the British think tank Chatham House, said that Mizintsev was a senior figure, but that he had spent much of the past decades in jobs that were "effectively administrative - not just staff posts, but running headquarters, command posts, coordination centers."

"So he's different from the other prominent senior Russian commanders who have mostly had operational on-the-ground experience in Syria," Giles wrote in an email.

The United States has not imposed sanctions on Mizintsev. However, Britain's Foreign Office said the Colonel-General said he was known "known for using reprehensible tactics, including shelling civilian centers in both Aleppo in 2015-16 and now in Mariupol - where atrocities are being perpetuated against Ukrainian people."

The destruction of Mariupol has drawn comparisons with the siege of Aleppo in 2016, when Russian forces helped Syrian President Bashar Assad crush rebels there in an eight-month campaign that featured the use of cluster bombs, chemical weapons and other banned munitions, in addition to heavy shelling and conventional airstrikes.

According to the Russian Ministry of Defense's website, Mizintsev headed a ministerial coordination body responsible for the return of refugees during the Syrian war.

Mizintsev has been named as Russia's point man in an agreement with the Ukrainians for a humanitarian cease-fire to allow beleaguered residents of Mariupol to evacuate and critical aid deliveries to enter the city, where more than 100,000 people are estimated to remain trapped.

The cease-fire is "purely for humane purposes," he said at a briefing Wednesday, the Interfax news agency reported.

But Sergey Bratchuk, a spokesperson for the Ukrainian military in Odesa, wrote on Facebook on March 23 that Mizintsev was "personally" controlling the military operation in Mariupol, adding that the Russian leader had ordered his troops to bomb a maternity hospital and other civilian targets.

The Ukrainian government has released audio that it claimed was an intercepted recording of Mizintsev berating and threatening Russian soldiers for their appearance. The Washington Post could not verify the authenticity of the recording.

Ukraine's former Ambassador to Austria, Olexander Scherba, shared the clip on Twitter on March 23, describing Mizintsev as "the butcher of Mariupol." Sherba later told The Washington Post that he did not know of Mizintsev before the war, adding that he was "not an expert" on the Russian army.

Giles, the Chatham House expert, said that Mizintsev was a surprising figure to take the brunt of blame for the attack on Mariupol, given his background.

But that didn't necessarily mean he wasn't responsible, Giles said. "I find it hard to believe that given the visibility into Russian operations that the U.S. and the U.K. have proven through their intelligence disclosures, that accusation can't be based on something substantial."

The relative anonymity of the Russian colonel-general was apparent in Russia too, according to Andrei Soldatov, a Russian investigative journalist who studies the country's security services. But that was nothing unusual, he said.

"He is not a big name in Russia," Soldatov said, adding that Russian Defense Minister Sergei Shoigu was really the only military leader known to the public. He was also popular - a fact that could explain why Russian President Vladimir Putin is cautious of firing the more senior official, Soldatov added.

Shoigu disappeared for 12 days this month, before returning to public life as abruptly as he departed on Saturday. His absence sparked widespread speculation about Russia's military leadership amid a war that is widely perceived to have gone badly.

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